My 5 in 1 has failed too. Temperature is stuck at -40. Replaced batteries, reset acu link, and still stuck at -40.
I was able to fix mine. Both temperature and dew point were stuck at minus 40. I first took apart the unit, didn't see any unusual dirt buildup, but wiped it down anyway. Reassembled, and still did not work. So thinking the sensor somehow got messed up, I decided to see if I could "jump start" it. Took a misting water bottle and sprayed lightly a few times from the bottom into the unit. Sure enough, within 15 minutes, the unit starting reporting again. It was running too high with dew point and humidity, but once it evaporated, it started reading correctly. I don't know how long that will last, but so far so good.